The present invention relates generally to nuclear magnetic resonance imaging systems and more particularly with the manner in which information is obtained from a localized point of interest in a specimen.
Nuclear magnetic resonance (NMR) imaging has had significant success as a non-invasive medical diagnostic tool, in that cross-sectional pictures through the human body can be obtained without exposing the patient to the risk of physiological harm attendant with the use of X-rays. Further, NMR imaging enables considerably more information to be obtained as to tissue state in internal organs, as compared to X-rays. For example, changes in the NMR relaxation time parameters T.sub.1 and T.sub.2, from point to point in a specimen can be used to discriminate between healthy and diseased tissue.
The basic principle of NMR can be appreciated by considering, for example, hydrogen nuclei associated with water in a tissue specimen. Such nuclei have an angular momentum arising from their inherent property of rotation, or spin. Since nuclei are electrically charged, the spin corresponds to a current flowing about the spin axis, which in turn generates a small magnetic field. Each nucleus has a magnetic moment, a dipole, associated with it. In the absence of an external magnetic field, the magnetic dipoles of the spinning nuclei will point in random directions.
When a static magnetic field is applied, the nuclei tend to align themselves with the field's lines of force, either with or against the field. The two orientations have slightly different energies, and the net difference between the upper and lower energy states will make the magnetization vector precess around a direction parallel to the applied magnetic field, in a manner analogous to a spinning top which precesses, or wobbles, under the influence of a gravitational field. This direction conventionally defines the Z axis.
If electromagnetic energy is now applied to the specimen at a frequency which matches the natural precessional frequency of the specimen, energy will be absorbed by the nuclei, causing the precessing bulk vector to tip further away from the Z axis. In turn, the increased tipping of the precessing bulk vector which results from the nuclear magnetic resources will increase the electromagnetic radiation from the nuclei at its resonant frequency, and such radiation can be detected by a receiver tuned to that frequency. The detected signal will have a magnitude proportional to the number of resonant nuclei in the tissue specimen.
A simple mathematical relationship links the resonance, or precession, frequency to the magnitude of the externally applied static magnetic field. The resonance frequency is equal to the strength of the field multiplied by the gyromagnetic ratio, which is unique for each nuclear species of nonzero spin. For hydrogen nuclei (protons) in a magnetic field of one tesla (10,000 gauss), the resonance frequency is 42.57 megahertz (MHz). For nuclei of the isotope phosphorous 31 in the same field, the resonance frequency is 17.24 MHz; for nuclei of sodium 23 it is 11.26 MHz. These frequencies are in the radiofrequency band of the electromagnetic spectrum. Such frequencies, far below those of X-rays or even visible light, have no known adverse effects on molecules of living systems.
In use as a diagnostic tool, information as to the nuclear concentration and relaxation parameters at many different points throughout a specimen are needed so that an image can be created for visual observation and study. The resolution of the image will depend, of course, on the number of discrete points in the specimen for which discrete information as to the nuclei concentration is obtained.
Conventionally the specimen is placed in a strong magnetic field and perturbing coils are used to create field gradients so that the field strength at a single plane through the specimen will have the same value throughout that plane. In principle, the specimen is irradiated with radio frequency energy of the proper frequency so that all of the nuclei of interest at that plane will resonate and produce a signal proportional to the total number of nuclei of that signal plane and dependent on their relaxation parameters. By a change of the currents in the perturbing coils, the signal plane is rotated about an axis and translated in space to obtain additional information about the nuclei in the different positions of the signal plane. Some systems use different frequencies to measure different planes simultaneously. Information as to each plane position and total nuclei concentration for that plane position are entered into a computer. After a full scan, the computer will solve the large number of simultaneous equations so that the nuclei characteristics at the points of intersection of the signal planes can be ascertained so that two dimensional images can then be created showing the nuclei characteristics at all the points in a desired plane through the specimen, or so that three dimensional images can be created on a cathode ray tube.
NMR imaging has been a successful and accurate method of measuring some tissue characteristics and thereby diagnosing the location of tumors, organ and irregularities in research animals and humans.
However, there are drawbacks to conventional NMR imaging systems. Good spatial resolution requires large amounts of scanning time, which is limited by the difficulty of immobilizing the patient or region to be scanned. This limitation is due in part to the time required to collect numerous configurations of one-dimensional signals that are needed to successfully map the region of interest. Since conventional tomographic methods portray density distributions on slices through a three dimensional specimen, this information is given in one or two dimensions. As a consequence powerful computers are needed to make the complex calculations that construct the region in three dimensions. If less powerful computers are used, a great deal of computer time is needed. Further, the conventional imaging systems are not as useful for imaging a rapidly fluctuating tissue, such as a pulsating heart, because of the movement of the tissue during the length of time required for a full scan.
Attempts have been made to obtain three-dimensional data directly, but very complicated modulations of the magnetic field, and/or of the frequency of the applied radio frequency energy, are required to determine the characteristics of resonating nuclei per unit volume at a single discrete point where spatial location is ascertainable. These systems are complex because localization of the region in interest depends on how the signal evolves in time. Further, these systems provide only a limited spatial resolution.